1. Technical Field
This invention relates to a Vertical-Cavity Surface-Emitting Laser (hereinafter referred to as VCSEL), a module, an optical transmission device, an optical transmission system, a free space optical communication device and a free space optical communication system.
2. Related Art
In recent years, in technical fields such as optical communication or optical storage, there has been a growing interest in VCSELs. VCSELs have excellent characteristics which edge-emitting semiconductor lasers do not have. For example, a VCSEL has a lower threshold current and consumes less power. With VCSELs, a round light spot can be easily obtained, and evaluation can be performed while they are on a wafer, and light sources can be arranged in two-dimensional arrays. With these characteristics, demands for VCSELs as light sources have been expected to grow especially in the communication field.
In a VCSEL of a related art, to improve the lowering in optical output at a high temperature, it has been proposed that the band gap of the AlGaAs layer having a high refractive index (low-Al-composition) and formed near the active layer is made greater, and the impurity doping concentration near the active layer is lowered. However, in such a method of the related art, carriers may not be sufficiently diffused to a center portion of the current confining region at a low temperature because the impurity doping concentration near the active layer is low. In addition, if the impurity doping concentration in the entire region apart from the active layer is increased so that carriers are sufficiently diffused at a low temperature, free carrier absorption in that region becomes greater, which lowers optical output.